DECIDUAL MEMBRANES 



371 



tact with which it does not clot. Elsewhere in the body, except in retic- 

 ular tissue, blood clots on escaping from the endothelial tubes. Toward 

 the uterine cavity, however, there is a clot which completes the encapsula- 

 tion of the chorionic vesicle in the mucosa. The mucous membrane itself 

 later passes entirely around the vesicle as shown in Fig. 371, A. 



The greater part of the mucosa of the uterus becomes cast off at the end 

 of pregnancy; thus it forms a membrana decidua, which may be subdivided 

 into three parts (i) the decidua basalis (or serotina) on which the im- 

 planted chorionic vesicle rests, and which forms the maternal part of the 

 placenta; (2) the decidua capsularis (or reflexa) which spreads over the 

 part of the vesicle which is toward the uterine cavity; and (3) the decidua 

 vera, which lines the remainder of the uterus. These subdivisions of the 

 decidua are indicated in Fig. 371, A. 



FIG. 371. THE UTERUS AND DECIDUAL MEMBRANES IN EARLY PREGNANCY, A, AND IN LATE PREGNANCY, 



B. THE CORD HAS BEEN CUT AND THE EMBRYO REMOVED FROM B. 

 am., Amnion; am. c., amniotic cavity; c., cervix; ch., chorion; c. u., cavity of the uterus; d. b., decidua 



basalis;;d. c., decidua capsularis: d. v., decidua vera; m., amnion and chorion laeve drawn as one 



line; pi., placenta; u. c., umbilical cord; y. s., yolk-sac. 



Soon after the ovum becomes implanted, the chorion ceases to be 

 uniformly covered with villi. The villi toward the decidua basalis elon- 

 gate and branch freely, producing the shaggy chorion frondosum; this is the 

 embryonic portion of the placenta. As the chorionic vesicle enlarges, the 

 villi directed away from the wall of the uterus, toward the decidua cap- 

 sularis, become shorter and disappear, so that a large portion of the 

 chorion becomes smooth the chorion lave. Usually the umbilical cord 

 passes to a nearly central position in the chorion frondosum; rarely it has 

 a "marginal attachment" at the periphery of the frondosum, and it may 

 have a "velamentous insertion" in the adjacent part of the chorion laeve, 

 through which the umbilical vessels then extend to the frondosum. 



With the growth of the embryo, which fills and distends the uterine 

 cavity, the decidua capsularis becomes thin, degenerates, and is resorbed, 



